Spin-transfer torque magnetic random access memory with perpendicular magnetic anisotropy multilayers

ABSTRACT

The present invention is directed to a spin transfer torque magnetic random access memory (STTMRAM) element comprising a composite free layer including one or more stacks of a bilayer unit that comprises an insulator layer and a magnetic layer with the magnetic layer having a variable magnetization direction substantially perpendicular to a layer plane thereof; a magnetic pinned layer having a first fixed magnetization direction substantially perpendicular to a layer plane thereof; a tunnel barrier layer formed between the composite free layer and the magnetic pinned layer; and a magnetic fixed layer coupled to the magnetic pinned layer through an anti-ferromagnetic coupling layer. The magnetic fixed layer has a second fixed magnetization direction that is substantially perpendicular to a layer plane thereof and is substantially opposite to the first fixed magnetization direction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of the commonly assignedapplication bearing Ser. No. 14/657,608 filed on Mar. 13, 2015, entitledSPIN-TRANSFER TORQUE MAGNETIC RANDOM ACCESS MEMORY WITH PERPENDICULARMAGNETIC ANISOTROPY MULTILAYERS,” which is a continuation of thecommonly assigned application bearing Ser. No. 13/225,338 filed on Sep.2, 2011, entitled “SPIN-TRANSFER TORQUE MAGNETIC RANDOM ACCESS MEMORYWITH PERPENDICULAR MAGNETIC ANISOTROPY MULTILAYERS,” which claimspriority to a previously-filed U.S. provisional application, U.S.Application No. 61/382,815, entitled “SPIN-TRANSFER TORQUE MAGNETICRANDOM ACCESS MEMORY WITH PERPENDICULAR MAGNETIC ANISOTROPYMULTILAYERS”, filed on Sep. 14, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spin-transfer torque (STT) magneticrandom access memory (MRAM), and, more particularly, to an STTMRAMelement having magnetic tunnel junctions (MTJs) with ferromagneticmultilayers whose magnetization is oriented perpendicular to the planeof the substrate, and having lower programming current density whilemaintaining higher thermal stability.

2. Description of the Prior Art

Magnetic random access memory (MRAM) is a type of non-volatile memory inwhich magnetization of magnetic layers in MTJs switches between parallel(corresponding to a low resistance state) and anti-parallel(corresponding to a high resistance state) configurations. One type ofMRAM is spin-transfer torque magnetic random access memory (STTMRAM)where switching occurs through the application of spin polarized currentacross the MTJ during programming.

STTMRAM has significant advantages over magnetic-field-switched (toggle)MRAM, which has been recently commercialized. The main hurdlesassociated with field-switched MRAM are its more complex cellarchitecture with high write current (currently in the order ofmilliamps (mA)) and poor scalability attributed to the process used tomanufacture these devices. That is, these devices cannot scale beyond 65nanometer (nm) process node. The poor scalability of such devices isintrinsic to the field writing methods. The current generated fields towrite the bits increase rapidly as the size of the MTJ elements shrinks.STT writing technology allows directly passing a current through theMTJ, thereby overcoming the foregoing hurdles and resulting in muchlower switching current (in the order of microamps (uA)), simpler cellarchitecture which results in a smaller cell size (for single-bitcells), reduced manufacturing cost, and more importantly, improvedscalability.

One of the challenges for implementing STT is a substantial reduction ofthe intrinsic current density to switch the magnetization of the freelayer while maintaining high thermal stability, which is required forlong-term data retention. Minimal switching (write) current is requiredmainly for reducing the size of select transistor of the memory cell,which is typically coupled in series with MTJ, because the channel widthof the transistor is proportional to the drive current of thetransistor. It is understood that the smaller the STT current, thesmaller the transistor size, leading to a smaller memory cell size. Asmaller current also leads to smaller voltage across MTJ, whichdecreases the probability of tunneling barrier degradation andbreakdown, ensuring a high write endurance of the MTJ cell. This isparticularly important for STTMRAM, because both sense and writecurrents are driven through MTJ cells.

One of the efficient ways to reduce the programming current in STTMRAMis to use an MTJ with perpendicular anisotropy. Incorporation ofconventional perpendicular anisotropy materials, such as FePt, intoSTTMRAM causes a high damping constant, leading to undesirably highswitching current density. Furthermore, during manufacturing,conventional higher ordering transformation temperature required forforming L10 order structure could degrade the tunnelingmagneto-resistance (TMR) performance and make MTJ deposition processmore demanding and complicated (such as elevated substrate temperaturesduring MTJ film deposition).

Prior art techniques rely on intrinsic perpendicular anisotropy of theFe-rich CoFeB alloys, and on the anisotropy from the interface with themain MgO barrier. Having a single magnetic layer is limiting however.The layer has to be not too thin. Otherwise, it will becomesuper-paramagnetic. It cannot be too thick either. Otherwise, it willbecome a layer with in-plane anisotropy. In this small parameter space,one has to make the coercive fields for the pinned and free layers asfar apart as possible. Low thermal stability (>40 required) is also anissue and it may be necessary to increase this parameter to 50-60 for areliable memory product. Thermal stability is defined by KuV/k_(B)T,where Ku represents magnetic anisotropy constant, V represents thevolume of the free layer, k_(B) is Boltzmann's constant and T representstemperature.

Thus there is a need to scale the perpendicular anisotropy of the freelayer according to its effective magnetic thickness.

There is also a need for an STTMRAM element having an MTJ withperpendicular magnetic anisotropy material(s) with a simple filmmanufacturing process and an optimal combination of saturationmagnetization (Ms) and anisotropy constant (Ku) to lower the dampingconstant and the magnetic anisotropy of the MTJ yielding a lower STTswitching current density while maintaining high thermal stability andhigh TMR performance.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa method and a corresponding structure for a magnetic memory device thatis based on current-induced-magnetization-switching having reducedswitching current in the magnetic memory.

Briefly, an embodiment of the present invention includes a spin-transfertorque magnetic random access memory (STTMRAM) element that has a pinnedlayer having a first magnetization that is substantially fixed in onedirection and formed on top of a substrate, and a tunnel barrier layerformed upon the pinned layer, and a composite free layer having a secondmagnetization that is switchable in two directions and formed upon thetunnel barrier layer. The composite free and pinned layers are made ofmultilayers with magnetic layers alternating with nonmagnetic insulatinglayers, such as aluminum nitride (AlN) and magnesium oxide (MgO). Themagnetization direction of each of the composite free layer and pinnedlayer being substantially perpendicular to the plane of the substrate.During a write operation, a bidirectional electric current is appliedacross the STTMRAM element switching the second magnetization betweenparallel and anti-parallel states relative to the first magnetization.In some embodiments, the thermal stability of the STTMRAM is 50-60,making for a reliable memory product.

These and other objects and advantages of the present invention will nodoubt become apparent after having read the following detaileddescription of the various embodiments illustrated in the severalfigures of the drawing.

IN THE DRAWINGS

FIG. 1 shows the relevant layers of STTMRAM element 100, in accordancewith an embodiment of the present invention.

FIG. 1A shows further details of one of the layers of the element 100,in accordance with an embodiment of the present invention.

FIG. 2 shows the relevant layers of STTMRAM element 300, in accordancewith another embodiment of the present invention.

FIG. 3 shows the layers of STTMRAM element 400, in accordance withanother embodiment of the present invention.

FIG. 4 shows the relevant layers of STTMRAM element 500, in accordancewith another embodiment of the present invention.

FIG. 5 shows the relevant layers of STTMRAM element 600, in accordancewith another embodiment of the present invention.

FIG. 6 shows the relevant layers of STTMRAM element 700, in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

In accordance with an embodiment of the present invention, aspin-transfer torque magnetic random access memory (STTMRAM) elementincludes a magnetic tunnel junction (MTJ) having a pinned layer and afree layer wherein the magnetic orientation of pinned layer and freelayer is substantially perpendicular to the plane of the layers. Theselection of the magnetic multilayers for use in each of the pinnedlayer and the free layer, as disclosed herein, advantageously reducesthe switching current density, and increases reliability in singledomain switching. In an exemplary embodiment, the switching currentdensity is less than 1 mega amp per centimeter squared (MA/cm²) whilemaintaining high thermal stability. Thermal stability is defined byKuV/k_(B)T. Ku represents magnetic anisotropy constant, V represents thevolume of the free layer, k_(B) is Boltzmann's constant and T representstemperature. In an exemplary embodiment, high thermal stability isgreater than 40.

In one embodiment, the pinned layer and the free layer each is made ofmultilayers with the multilayers being made of a different number ofrepeated layers or repeated combination of layers. In some embodiments,the repeated layer is a bilayer made of a combination of a nonmagneticinsulator layer (A) and a magnetic layer (B). (A) or ‘A’ can be made ofone or more nonmagnetic insulating material from the class of oxides,such as magnesium oxide (MgO), aluminum oxide (Al₂O₃), zinc oxide (ZnO),titanium oxide (TiO₂), strontium oxide (SrO), ruthenium oxide (RuO),silicon oxide (SiO₂), zirconium oxide (ZrO₂), hafnium oxide (HfO₂),tantalum oxide (TaO); and nitrides, such as aluminum nitride (AlN),titanium nitride (TiN), tantalum nitride (TaN), silicon nitride (SiN),zirconium nitride (ZrN), or mixed of oxide and nitrides, such as but notlimited to aluminum oxide nitride (AlON) and other suitable oxide and/ornitride mixes. Layer (B), or ‘B’, can be any of the following magneticmaterials: iron (Fe), iron-rich cobalt iron (CoFe) alloys, iron-richnickel iron (NiFe) alloys, iron-rich CoFeNiB, iron-rich cobalt ironboron (CoFeB) alloys, iron-rich nickel iron boron (NiFeB) alloys,iron-rich iron oxide (FeOx), CoFeOx. Layer B can also in itself be amagnetic multilayer that contains a thin boron (B) absorptionnonmagnetic metal layer, which in some embodiments may be between 0.3 to1.0 nano meters (nm).

This absorption nonmagnetic metal layer can make multi-layerperpendicular anisotropy more thermally stable and helps produce highereffective perpendicular anisotropy in the magnetic layer with promotinginterface anisotropy effect between layer A and layer B, in particular,during thermal treatment. Material of this thin (boron) absorption layercan be a single element layer, such as Ta, Ti, Ru, and can also be acomposite material composed of one or more of the following materials:Ta, Ti, Ru, in addition to one or more of the following materials: B,Co, Fe, Ni.

Referring now to FIG. 1, the relevant layers of an STTMRAM element 100are shown, in accordance with an embodiment of the present invention.STTMRAM element 100 is shown to comprise: bottom electrode (BE) 101,underlayer(s) 102, pinned layer (or “fixed layer”) 103, tunnel (or“barrier” or “tunnel barrier”) layer 104, free layer (or “composite freelayer”) 105, cap layer 106 and top electrode (TE) 107 with each of thepinned layer 103 and/or the free layer 105 comprising a multilayer 10.An exploded view of the multilayer 10 is shown under the STTMRAM element100. It is understood that the layer 103 and the layer 105 are each madeof the multilayer 10.

As with other embodiments shown in subsequent figures herein, abidirectional electrical current is applied either at 111 or at 113 tothe element 100, during operation thereof, causing the layer 105 toswitch its magnetization from parallel to anti-parallel or vice versarelative to that of the layer 103 to store a bit of information (or alogical state) in the element 100.

The BE 101 is typically formed on top of the substrate (not shown inFIG. 1) or on top of the metal layer such as Cu or tungsten (W), in caseof a CMOS starting wafer. The underlayer 102, which in one embodiment ismulti-layered and in another embodiment is a single layer, is formed ontop of the BE 101. The pinned layer 103 is formed on top of theunderlayer 102. The tunnel barrier layer 104 is formed on top of thepinned layer 103. The free layer 105 is formed on top of the tunnellayer 104. The cap layer 106 is formed on top of the free layer 105 andthe TE 107 is formed on top of the cap layer 106.

The pinned layer 103 and the free layer 105 each comprises a compositemultilayer structure with multiple repeats of a basic bilayer unit 203shown by the multilayer 10 of FIG. 1. The multilayer 10 is shown made ofa magnetic layer 201 and a nonmagnetic insulating layer 202, describedhereinabove as nonmagnetic insulator layer (A) and a magnetic layer (B),respectively. This basic bilayer unit 203 is designed so as to enhancethe perpendicular anisotropy in the magnetic layer 201. This enables themagnetic moment in the layer 201 to have a preferred orientation that isperpendicular to the layer plane. The magnetic layers, in close contactwith the tunnel barrier layer 104, can be of different materials than inother magnetic layer repeats.

The perpendicular anisotropy comes about by a combination of chemicalcomposition and interface effects. In the embodiment of FIG. 1,iron-rich materials are used in combination with an insulating material,such as MgO. This is known to lead to perpendicular anisotropy in asingle magnetic layer. However, the parameter space for desirablecharacteristics is limited in the case of a single layer. One has tomake the magnetic layer thin enough (typically below 3 nm) so thatinterface anisotropy energy is enough to bring the magnetization to anout-of-plane preferred direction. An iron-rich CoFeB alloy magneticlayer has perpendicular anisotropy when it is typically below 1.3 nm inthickness. Making this magnetic layer too thin poses problems with thelayer becoming superparamagnetic (thermally unstable) or evennonmagnetic at room temperature. The pinned layer 103 and the free layerneed to have different coercive fields, where coercive field refers toas the magnetic field applied to change the orientation of magnetizationfrom one direction to the other direction (related to the magneticanisotropy), so that one switches its magnetization easier than theother, hence the names “pinned” and “free”. For a single layer this isachieved by varying the thickness of the magnetic layer, but there isnot a lot of room to improve because of limited thickness range. Inaddition, the thermal stability is proportional to the interface energyand having only one interface, or two at most, is limiting.

In the embodiment of FIG. 1, multiple interfaces and magnetic layers arerepeated to make up for the limited useful thickness range in the singlemagnetic layer. In essence a multilayer approach is utilized to scale upthe perpendicular anisotropy without the penalty of reduced effectivemagnetic thickness. The bilayer basic unit has the insulating layerthickness, in some embodiments, in the range of 0.3-3 nm and theferromagnetic layer thickness, in some embodiments, is in the range of0.3-6 nm. In the thinner insulating thickness range there can bepinholes in the insulating layer through which the ferromagnetic layerscan be magnetically coupled. This makes the entire multilayer structureeven more stable against thermal fluctuations of the magnetizationdirection. For thicker insulating layer, the magnetic layers aredecoupled, so a thicker magnetic layer is generally preferred.

As previously noted, the layer 201 is magnetic layer and 202 is aninsulator or “insulating” layer. The pair of layers 201 and 202 can alsoform a repeating structure, basic bilayer unit 203 that repeats in athickness (or vertical) direction to form a multi-layer structure. Insome embodiments, the multilayer structure advantageously ends with oneof the magnetic layers 201 being adjacent and in direct contact with thebarrier layer 104 of an element 100. The basic bilayer unit 203 can berepeated an integer, ‘n’, number of times with n being one or greater.

FIG. 1A shows further details of the layer 201, in accordance withanother embodiment of the invention. The layer 201 is shown to include amagnetic layer 704 and a magnetic layer 705 separated by a non-magneticboron absorption layer 706, where 704 and 705 may or may not beidentical in composition depending on the effective perpendicularanisotropy and the effective magnetic moment optimization requirementfor layer 201. All these iron-rich alloys contain more than 50% iron inatomic percentage. These alloys may be doped with a small atomicpercentage (typically below 15 at. %) of other nonmagnetic elements, Zand Y, in order to obtain the desired Ku (anisotropy constant) and Ms(saturation magnetization) combination for reducing the dampingconstant, magnetization (Ms), and for reducing the distribution ofperpendicular magnetic anisotropy. Z can be one or more of thesematerials: boron (B), phosphorous (P), carbon (C), nitrogen (N) and Yrepresents any of the materials: tantalum (Ta), titanium (Ti), niobium(Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver(Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb),hafnium (Hf), bismuth (Bi), molybdenum (Mo), or rhodium (Rh). Inaddition, the magnetic layer B can also be comprised of multi-layer ofmagnetic materials such as described above.

In another embodiment of the invention, the magnetic layer 201 that isadjacent to the barrier layer 104 is made of CoFe and CoFeB alloy havinggreater than 50 at. % of Fe, or Fe—ZY alloy having less than 15 at. % ofZ, where Z is one or more of the following materials: boron (B),phosphorous (P), carbon (C), nitrogen (N) and Y represents any of thematerials: tantalum (Ta), titanium (Ti), niobium (Nb), zirconium (Zr),tungsten (W), silicon (Si), copper (Cu), silver (Ag), aluminum (Al),chromium (Cr), tin (Sn), lead (Pb), antimony (Sb), hafnium (Hf), bismuth(Bi), molybdenum (Mo), or rhodium (Rh).

All these iron-rich alloys contain more than 50% iron in atomicpercentage. These alloys may be doped with a small atomic percentage(typically below 15 at. %) of other nonmagnetic elements, Z and Y, inorder to obtain the desired Ku (anisotropy constant) and Ms (saturationmagnetization) combination for reducing the damping constant,magnetization (Ms), and therefore for reducing the distribution ofperpendicular magnetic anisotropy. Z can be one or more of thesematerials: boron (B), phosphorous (P), carbon (C), nitrogen (N) and Yrepresents any of the materials: tantalum (Ta), titanium (Ti), niobium(Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver(Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb),hafnium (Hf), bismuth (Bi), molybdenum (Mo) or rhodium (Rh). Inaddition, the magnetic layer B can also be comprised of multi-layer ofmagnetic materials such as described above. In a preferred mode, themagnetic layer adjacent to the “barrier” layer is comprised of CoFe andCoFeB alloy having greater than 50 at. % of Fe, or Fe—ZY alloy havingless than 15 at. % of Z and Y, where Z is one or more of the followingmaterials: boron (B), phosphorous (P), carbon (C), nitrogen (N) and Yrepresents any of the materials: tantalum (Ta), titanium (Ti), niobium(Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver(Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb),hafnium (Hf), bismuth (Bi), molybdenum (Mo), or rhodium (Rh).

The damping constant and the magnetic perpendicular anisotropy of thepinned layer are designed to be much larger than that of the free layer.This can be realized with a larger number of repeats of the basic unitused in the pinned layer, leading to an increased perpendicularanisotropy and coercive field.

As previously stated, the layer 706, in some embodiments, is a singleelement layer, and made of material, such as Ta, Ti, Ru, and in otherembodiments, it is made of a composite material from one or more of thefollowing: Ta, Ti, Ru, in addition to one or more of the followingmaterials: B, Co, Fe, Ni.

FIG. 2 shows an STTMRAM element 300, in accordance with anotherembodiment of the invention. The element 300 is shown to include the BE101, the underlayer 102, a perpendicular magnetization pinned layer(PM-PL) 301, a barrier layer 104, a perpendicular magnetization freelayer (PM-FL) 302, the cap layer 106, and the TE 107. Each of the layers301 and 302 is made of perpendicular anisotropy multilayer structures asdescribed in FIG. 1. The layer 301 has a larger number of repeats of themultilayer structure (multilayer 10) to enhance its coercive field andmake its magnetization direction very stable. The free layer 105 of theelement 100 typically has a fewer number of repeated bilayer units thanthe layer 302 to enable easier switching with an electric current thatis polarized by the stable pinned layer. The free layer 105 however, hasa suitable number of repeated bilayer units to make it stable againstthermal fluctuations that might cause its magnetization to flip. Thecapping layer 106 can enhance the perpendicular anisotropy of the freelayer PM-FL 302 and can be any of the materials: Mn containing alloyssuch as PtMn, NiMn, IrMn, FeMn, or Ru or Ta. The term “PM”, as usedherein, is an acronym for “perpendicular multilayer”. The term “PL”, asused herein, is an acronym for “pinned layer”.

FIG. 3 shows an STTMRAM element 400, in accordance with anotherembodiment of the invention. The STTMRAM element 400 is analogous to theelement 100 except that it has a multi-layered pinned layer, formedbetween the underlayer 102 and the barrier layer 104. Namely, themultilayer structure of the pinned layer of the element 400 includes aperpendicular magnetic pinned layer (PM-PL) (also referred to herein as“composite magnetic layer”) 402 formed on top of the underlayer 102, aspacer layer 403, formed on top of the layer 402, and a PM-PL 404,formed on top of the layer 403. Optionally, an anti-ferromagnetic layer(AFM) 401 is shown formed on top of the layer 102 and below the PM-PL402. Accordingly, the pinned layer of the STTMRAM element 400 is made upof the PM-PL 402, the layer 403, and the PM-PL 404 (also referred toherein as “composite magnetic layer”).

In some embodiments, the spacer layer 403 is non-magnetic and made of Ruand acts to make the magnetization directions of the PM-PL 402 and PM-PL404 anti-parallel relative to each other. The PM-PL 402 and PM-PL 404,each has either the multilayer 10 shown in FIG. 1, or other embodimentsdescribed herein.

In those embodiments using the AFM 401, the pinned layer of the element400 includes the AFM 401. In these embodiments, the AFM 401 is used topin strongly the magnetization of the layer PM-PL 402 in a givendirection, thus making the pinned layer of the element 400, in FIG. 3,harder to switch.

The PM-PL 404 and PM-PL 402 of the element 400, being coupled to eachother in an anti-parallel direction relative to each other through athin nonmagnetic conducting spacer layer (such as Ru), i.e. layer 403,has the effect of reducing the magnetic dipolar stray field acting onthe free layer 105. This is important for controlling symmetrical writecurrents (making the write current I^(ap-p) value from antiparallelresistance state to parallel state closely equal to that of the writecurrent I^(a-ap) from parallel resistance state to antiparallel state)in a memory device, such as the STTMRAM elements of the variousembodiments of the invention.

FIG. 4 shows an STTMRAM element 500, in accordance with yet anotherembodiment of the invention. The element 500 is analogous to the element100 except that its free layer is made of multiple layers. That is, thefree layer of the element 500 is formed of the perpendicular magneticfree layer (PM-FL) (also referred to herein as “composite magneticlayer”) 501, the spacer layer 502, and the PM-FL (also referred toherein as “composite magnetic layer”) 503 with the PM-FL 501 beingformed on top of the barrier layer 104, the spacer layer 502 beingformed on top of the PM-FL 501, and the PM-FL 503 being formed on top ofthe layer 502. The cap layer 106 is formed on top of the PM-FL 503. Thefree layer of the element 500 has a synthetic coupling structure.

The PM-FL 501 and the PM-FL 503, are each made of magnetic material,such as those listed hereinabove relative to the PM-FL 302 of theelement 300, in some embodiments of the invention. The PM-FL1 501 andPM-FL2, 503 have the multilayer structure shown in FIG. 1 and subsequentembodiments described herein.

In some embodiments, the layer 502 is made of any of the materialruthenium (Ru), chromium (Cr), or magnesium oxide (MgO). Themagnetizations in the PM-FL 501 and PM-FL 503, each have perpendicularanisotropy (preferred directions) and can be coupled either parallel oranti-parallel through the layer 502. This type of structure reduces theswitching current while maintaining thermal stability.

FIG. 5 shows an STTMRAM element 600, in accordance with anotherembodiment of the invention. The element 600 is analogous to the element500 except for its free layer, which includes additional layers. Thatis, the free layer of the element 600 includes an additional spacerlayer 604 and a free sub-layer 605 as a part of its free layer, whichoffer an enhancement over previous embodiment herein of the free layerstructure.

The free layer of the element 600 is shown to include a PM-FL 601, aspacer layer 602, formed on top of the PM-FL 601, a PM-FL 603, formed ontop of the layer 602, the spacer layer 604, formed on top of the PM-FL603, and the sub-layer 605, formed on top of the layer 604. The freelayer has a perpendicular anisotropy and is formed on top of the barrierlayer 104. The cap layer 106 is formed on top of the sub-layer 605.

In some embodiments, the layer 604 is made of non-magnetic material andthe sub-layer 605 is made of magnetic material and has an in-planeanisotropy. As current passes through the STTMRAM element 600, thein-plane polarized electrons coming from the sub-layer 605 help switchthe magnetization in the PM-FL 603.

In some embodiments, the PM-FL 601, the layer 602, and the PM-FL 603 areeach made of corresponding layers of the element 500. The layer 604 ismade of any of the materials: magnesium oxide (MgO), ruthenium (Ru) andTa (tantalum). The sub-layer 605 is made of any of the followingmaterials: CoFeB alloys, CoFe alloys, or CoFeB—X alloys, where X is anyof the elements Ti, or Ni. It is noted that “FL” as used herein is anabbreviation of the term “free layer”.

FIG. 6 shows an STTMRAM element 700, in accordance with anotherembodiment of the invention. The element 700 is analogous to the element300 except that it includes additional layers formed on top and bottomof the barrier layer 104. The element 700 is shown to include aninsertion layer 601, formed on top of the PM-PL 301, on top of which isformed the barrier layer 104, and an insertion layer 603, formed on topof the barrier layer 104. The PM-FL 302 is shown formed on top of theinsertion layer 603.

The insertion layers 601 and 603, alternatively, can be formed adjacentto the barrier layer of the other various embodiments shown anddiscussed herein to improve the tunnel magnetoresistance (TMR) of theSTTMRAM element in which they are added and further, to reduce theswitching current thereof, as experienced by the element 700. In element700, the insertion layers 601 and 603 may be considered a part of thebarrier layer of the element 700.

In some embodiment, the insertion layers 601 and 603 are each made ofany of the following materials: Co, Fe, CoX1, FeX1 and CoFeX1, withoptional materials X1 which can be boron, Ta, Ti, Hf, or Cr.

It is noted that the thickness of the lines shown in various figures ofthe embodiments, in addition to any of the sizes (widths) of the variouslayers shown in the figures of the various embodiments, in no wayreflect actual sizes or represent thickness or are relevant to thevarious embodiments shown and described herein.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A spin transfer torque magnetic random accessmemory (STTMRAM) element comprising: a composite free layer includingone or more stacks of a bilayer unit that comprises an insulator layerand a magnetic layer, said magnetic layer having a variablemagnetization direction substantially perpendicular to a layer planethereof, a magnetic pinned layer having a first fixed magnetizationdirection substantially perpendicular to a layer plane thereof; anon-magnetic tunnel barrier layer formed between said composite freelayer and said magnetic pinned layer; and a magnetic fixed layer coupledto said magnetic pinned layer through an anti-ferromagnetic couplinglayer, said magnetic fixed layer having a second fixed magnetizationdirection that is substantially perpendicular to a layer plane thereofand is substantially opposite to said first fixed magnetizationdirection.
 2. The STTMRAM element of claim 1, wherein saidanti-ferromagnetic coupling layer is made of ruthenium.
 3. The STTMRAMelement of claim 1, wherein said non-magnetic tunnel barrier is made ofmagnesium oxide.
 4. The STTMRAM element of claim 1, wherein saidinsulator layer of said bilayer unit is made of a magnesium oxidecompound, said magnetic layer of said bilayer unit comprises cobalt andiron.
 5. The STTMRAM element of claim 1, wherein said insulator layer ofsaid bilayer unit is made of an insulator material selected from thegroup consisting of aluminum oxide, zinc oxide, titanium oxide,strontium oxide, ruthenium oxide, silicon oxide, zirconium oxide,hafnium oxide, tantalum oxide, silicon nitride, and any combinationsthereof.
 6. The STTMRAM element of claim 1, wherein said magnetic layerof said bilayer unit further comprises first and second magneticsublayers separated by a boron absorption layer, said first and secondmagnetic sublayers having said variable magnetization directionsubstantially perpendicular to layer planes thereof.
 7. The STTMRAMelement of claim 6, wherein said boron absorption layer is made of amaterial selected from the group consisting of Ta, Ti, Ru, and anycombinations thereof.
 8. The STTMRAM element of claim 6, wherein saidboron absorption layer is made of a material selected from the groupconsisting of Ta, Ti, Ru, Co, Fe, Ni, and any combinations thereof. 9.The STTMRAM element of claim 6, wherein at least one of said first andsecond magnetic sublayers is made of a CoFeB or CoFe alloy.
 10. TheSTTMRAM element of claim 1, wherein said magnetic pinned layer comprisesiron and cobalt.
 11. The STTMRAM element of claim 1, wherein saidmagnetic pinned layer further comprises first and second magneticsublayers separated by a boron absorption layer, said first and secondmagnetic sublayers having said first fixed magnetization directionsubstantially perpendicular to layer planes thereof.
 12. The STTMRAMelement of claim 11, wherein said boron absorption layer is made of amaterial selected from the group consisting of Ta, Ti, Ru, and anycombinations thereof.
 13. The STTMRAM element of claim 11, wherein saidboron absorption layer is made of a material selected from the groupconsisting of Ta, Ti, Ru, Co, Fe, Ni, and any combinations thereof. 14.The STTMRAM element of claim 11, wherein at least one of said first andsecond magnetic sublayers is made of a CoFeB or CoFe alloy.
 15. TheSTTMRAM element of claim 1, further comprising a magnetic insertionlayer formed between said composite free layer and said tunnel barrierlayer.
 16. The STTMRAM element of claim 15, wherein said magneticinsertion layer comprises iron.
 17. The STTMRAM element of claim 15,wherein said magnetic insertion layer is made of a magnetic metal oralloy selected from the group consisting of Co, Fe, CoFe, B, Ta, Ti, Hf,Cr, and any combinations thereof.
 18. The STTMRAM element of claim 1,further comprising a magnetic insertion layer formed between saidmagnetic pinned layer and said tunnel barrier layer.
 19. The STTMRAMelement of claim 18, wherein said magnetic insertion layer comprisesiron.
 20. The STTMRAM element of claim 18, wherein said magneticinsertion layer is made of a magnetic metal or alloy selected from thegroup consisting of Co, Fe, CoFe, B, Ta, Ti, Hf, Cr, and anycombinations thereof.